Noise reducer for amplitude modulation reception



- June 24, 1947. 2,422,976

NOI-SE REDUCER FOR AMPLITDE MODULATION RECEPTION M. G. NICHOLSON, JR

2 sheets-sneer 1 Filed June'ze, 1944 lune 24,1947. M. G. NICHOLSON, JR2,422,976

NOISE REDUCER FOR AMPLITUDE MODULATION RECEPTION Filed June 29, 1944 2sheets-sheet 2 /ZVB se Pase JWN INVENTOR.

Patented June 24, 1947 NOISE REDUCER FOR AIVIPLITUDE MODULATIONRECEPTION Madison G. Nicholson, Jr., Snyder, N. Y., assigner to ColonialRadio Corporation, Buffalo, N. Y.

Application June 2 9, 1944, Serial No. 542,739

` (ci. l25o-20) l Claims.

This invention relates to noise limiting circuits, and more particularlyto noise limiting circuits adapted and arranged for use in the receptionof amplitude modulated signals.

Various arrangements have been suggested in the past for limiting theresponse of receivers for amplitude modulated signals to noise of theimpulse type, such as that produced in the loud speaker by automobileignition, vacuum cleaner motors, electric Shavers, and the like; andthese generally, to the extent that they are operative, are based on theapplication of the principle of limiting, employing a tube or tubeswhich saturate at values corresponding to one hundred per centmodulation of the strongest signal to be received or in some cases onehundred per cent modulation of the signal actually being received. Inthe latter case the circuits limit the noise cccurring in the outputcircuit to that equivalent to one hundred percent modulation.

In accordance with my invention, I am able to still further reduce thenoise which remains even when such systems are operating properly, andwhich would otherwise pass through the limiting devices.

Among the objects of my invention may be mentioned the following:

To provide a circuit which will eliminate or largely reduce noise wouldotherwise be stronger than the signal.

To provide a,` system in which the noise impulse previously limited toone hundred per cent level are prevented from distorting the wave formof the audio currents except to a very minor degree.

To. provide a circuit which will maintain to a substantial degree thedesired wave form. of the audio signal inthe presence of noise whichwould otherwise greatly distort it, even though limited to the level ofone hundred per cent modulation.

To provide a radio receiver which is relatively immune to noise of theelectrical impulse type and which will operate with relative freedomfrom noise in the immediate neighborhood of other electrical apparatus,such as automobile ignition systems, electric Shavers, vacuum cleaners,and the like.

StillV other objects and advantages of my invention will` be apparentfrom the specification.

The features of novelty which I believe .to be characteristic of myinvention. are set forth with particularity in the appended claims. Myinvention itself, however, both as to its fundamental principles and asto its particular embodiments,

will best be understood by reference to the` specification andaccompanying drawing, in which Fig. 1 is a circuit diagram of apparatusin accordance with my invention, and

Fig. 2 is a series of curves illustrating the operation of the circuitin accordance with my invention.

Referring now more particularly to the drawing, my invention ispreferably applied just before the final detection of the receivedsignal. Thus I0 may represent the final coupling transformer from thelast intermediate stage of a superheterodyne receiver or the last T. R.F. stage of a T. R, F. receiver.

The transformer I may comprise primary coil I Ila shunted by tuningcondenser III'c and secondary coil Ib shunted by condenser Illd. Sincethe parts ofV the receiverprior to this point are well understood in theart and per' se are not part of this invention, they are not shown ordescribed. 'I'he signal supplied to the coil Ilb may be passed throughcondenser I I and impressed upon the rectifier diode I2, which maycomprise cathode I2c and anode I`2`a. The currents rectified by thediode I2 pass through resistances- I3 and I4 connected in series andhaving their upper end connected to the diode anode Ia and their lowerend to ground, as indicated. Y

Resistance III maybe shunted by by-pass condenser I5, which eliminatesfrom resistor I4 any radio frequency currents. From theV common pointofA resistances I3 and I4 a connection may be made to anode 26a ofrectier diode 2li, having also cathode Zfc. The latter may be connectedto ground throughcondenser 2,4, and may supply audio signals to theaudio frequency amplifier through condenser ZI, resistance 22, andvolume control resistance 23, all being connected in series n betweencathode 20c and ground. The audio fre- ,to ground through resistance25". From the anode 12a of diode I2` a connection may be made throughresistance I8 to anode 26a of diode 2li, which anode may be connected'to ground through condenser 2l.

An additional diode Zlimay be providedhaving its anode 29d connected tothe cathode Zlio and its cathode 29o connected through resistance 30 andcondenser 28 to the common point of condensers I6 and I1, and the end ofresistance I8, which is connected to anode 26a, may be connected throughresistance I9 to the common point of condenser 28 and resistance 30. Theoperation of this circuit will now be described.

Diode I2 is the signal demodulator and, as will be understood, producesa rectified current flowing through resistances I3 and I4 in a directionsuch as to make the end of resistance I 3, which is connected to theanode I2a. negative with respect to ground. Thus the potential of theanode I2a iluctuates With respect to ground, being at all times more orless negative, depending upon the strength of the signal, but never,under any circumstances, becoming positive.

At this point it should also be kept in mind, for aid in understandingthe operation of my invention, that electrical impulses of the noisetype likewise make the anode I2a negative with respect to ground to agreater or less degree, depending on the amplitude of the noise impulse,and never, under any circumstances, make it positive. This is clearlyshown in Fig. 2, in which the curve E (B) represents the audio wave formand is actually the potential difference between point B and ground.

It will be seen from this that the average value of EB .is negative andrepresents the axis of the modulation curve EB, which can nevel` goabove the zero line. For normal operation; that is. with no noise, areduced signal voltage is applied to the anode 20a of diode 20 throughthe voltage divider formed by resistances I3 and I4, which arepreferably chosen so that the voltage from point A to ground is aboutone-third that from point B to ground. So long as the anode 20a ispositive with respect to cathode 20cy diode 20 will act like a wireconnection and will pass the audio frequency signal through condenser 2Iand resistances 22 and 23. The potential of point A with respect toground is shown in curve EA.

In the absence of noise, anode 20a is positive with respect to cathode20c, since the latter derives its potential from point B throughresistances I8, I9, and 30, and diode 29, and point B is more negativethan point A,

Suppose, now, a short, sharp noise impulse is received, as indicated bythe peak N on curve EB. In the relatively short time during which suchpulse occurs,'the potential of point C will not have time to riseappreciably, because of the connection of point C to condenser 2l.However, the potential of point B will go sharply negative as indicatedin Fig. 2, and the potential of point A, as shown on the curve EA willalso go sharply negative by one-third of the value of the change inpotential of point B. This is sufficient to make anode 20a negative withrespect to cathode 20c, and thereby stops the flow of current throughdiode 20.

In this way noise pulses which would appear in the audio output wouldlimit to a iinite value (usually one hundred per cent modulation level),regardless of the magnitude of the incoming pulses above the criticallimiting value. Such a condition would be as indicated in curve EL, buteven these limited pulses are prevented by my invention as follows.

Since the voltage across condenser 2I will not follow the normal changesin Voltage at point B brought about by modulation, it is necessary tochoose such a ratio of resistances I3 and I4 that condenser 21 is keptcharged to a value just sufficient to accommodate one hundred per centmodulation of the incoming signal without the current through diode 20being reduced to zero.

While a noise pulse is being received, as already stated, diode 20 actsas an open circuit and the voltage at point D will begin to approach thevoltage at point C, the rate of approach being determined primarily bythe time constant of resistance 30 and condenser 24. Should this processcomplete itself, the resulting noise pulse in the output would have anamplitude equal to one hundred per cent modulation, as shown in curveEL. However, this process is not allowed to complete itself because thenoise pulse is rectied in the diode 26, producing a voltage which makespoint E more positive than point D, thus opening the circuit throughdiode 29.

Thus during the reception of noise pulses point D is isolated from allinput sources because both diodes 20 and 29 no longer pass current, andpoint D simply floats during the small time interval of the noise pulse.Diode 26 is biased to cut-off for all signals equal or less than normalmodulation peaks, this being determined by the ratio of condensers I6and I'I. The final or output wave then has the form shown in curve Eo,from which it will be seen that the only eiect of the noise impulse isto produce a small irregularity or dimple in the iinal audio wave form.

For the purpose of making the disclosure complete, but not by way oflimitation, the following table of values is given:

Resistauces Values Condcnsers Values 68,000 ohms 16 25 mmf 30,000 ohms.17 50 mm1 15,000 oh1ns 27 0.1 mi. 0.47 megohm 24 0.0005 mf 0.1 megohins.2l 0.01 mi 0.47 megohms 28 0.01 mf 2 megohms 15 40 mmf 22 0.1 megohmWhile I -have shown and described certain preferred embodiments of myinvention, it will be understood that modications and changes may bemade Without departing from the spirit and scope thereof, as will beclear to those skilled in the art.

In this application I have particularly pointed out and distinctlyclaimed the part, improvement, or combination which I claim as myinvention or discovery, and I have explained the principles thereof andthe best mode in which I have contemplated applying those principles soas to distinguish my invention from other inventions.

I claim:

l. In a radio receiver of the amplitude modulation type, in combination,a signal demoduu lator, a capacity charged thereby, an audio outputcircuit containing capacity, a, connection between said output circuitand said demodulator and a second connection between said output circuitand the first capacity, and means for interrupting both of saidconnections during the occurrence of noise pulses substantiallyexceeding the level ci" one hundred per cent positive modulation peaks.

2. In a radio receiver of the amplitude modulation type, in combination,a signal demodulator, a capacity charged thereby, an audio outputcircuit containing capacity, a connection between said output circuitand said demodulator and a second connection between said output circuitand the first capacity, and means comprising a pair of diodes forinterrupting both of said connections during the occurrence of noisepulses substantially exceeding the level of one hundred per centpositive modulation peaks.

3. In a radio receiver of the amplitude modulation type, in combination,a signal demodulator, a capacity charged thereby, an audio outputcircuit containing capacity, a connection between said output circuitand said demodulator and between said output circuit and the firstcapacity, and means comprising a pair of diodes and means formaintaining the anodes thereof negative with respect to their cathodesfor interrupting both of said connections during the occurrence of noisepulses substantially exceeding the level of one hundred per centpositive modulation peaks.

4. In a radio receiver of the amplitude modulation type, in combination,a signal demodulator, a capacity charged thereby, an audio outputcircuit containing capacity, a connection between said output circuitand said demodulator and between said output circuit and the rstcapacity, and means comprising a. pair of diodes and means formaintaining the anodes thereof negative with respect to their cathodesfor interrupting both of said connections during the occurrence of noisepulses substantially exceeding the level of one hundred per centpositive modulation peaks, having the anode of one and the cathode ofthe other connected together.

5. In a radio receiver of the amplitude modulation type, in combination,a signal demodulator, a capacity charged thereby, an audio outputcircuit containing capacity, a connection between said output circuitand said demodulator and between said output circuit and the firstcapacity, and means comprising a pair of diodes and means formaintaining the anodes thereof negative with respect to their cathodesfor interrupting both of said connections during the occurrence of noisepulses substantially exceeding the level of one hundred per centpositive modulation peaks, said last mentioned means comprising a thirddiode arranged to rectify noise pulses having an amplitude substantiallyexceeding the level of one hundred per cent positive modulation peaks.

6. In a radio receiver of the amplitude modulation type, in combination,a signal demodulator, a capacity charged thereby, an audio output'circuit containing capacity, a connection between said output circuitand said demodulator and between said output circuit and the firstcapacity, and means comprising a pair of diodes and means formaintaining the anodes thereof negative with respect to their cathodesfor interrupting both of said connections during the occurrence of noisepulses substantially exceeding the level of one hundred per centpositive modulation peaks, said last mentioned means comprising a thirddiode arranged to rectify noise pulses having an amplitude substantiallyexceeding the level of one hundred per cent positive modulation peaks,said last diode being biased to prevent operation in response tovoltages less than the level of one hundred per cent positive REFERENCESCITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,344,697 Hollingsworth Mar. 21,1944 2,129,027 Roberts Sept. 6, 1938 2,135,949 Roberts Nov. 8, 19382,172,922 Clay Sept. 12, 1939 2,192,189 Haffcke Mar. 5, 1940 2,208,398Simons July 16, 1940 2,208,953 Weber, Jr July 23, 1940 2,216,517Oosterhuis Oct. 1, 1940 2,231,371 Roberts Feb. 11, 1941

